Toilet "sucking" water from tub trap?

I have what I think is a weird problem. When I flush my toilet, there is a gurgling sound in my bath tub. When I run some water in the tub,and then flush, the noise stops, for a bit, (3/4 flushes) and starts again. Even more weird. this only happens on an intermittent basis. Sometimes it will go weeks without happening, then do it for a week. It seems that for some reason the toilet is sucking the water from the trap for the bath. I thought it might have something to do with the snow on the roof covering the vent stack but I just checked it from the ground) and it is open. Could there be a plug of snow inside the stack pipe that I can't see from the ground? (stack should be warm enough to prevent that?)

Would that cause an intermittent problem Terry? I have been reading here for an hour and came across threads that talk about dry traps(this one is used daily). I am kind of concerned now because I also read about "partial" clogging of the lines.

If it is siphoning, it is because the toilet is flushing "good", so it would not be a stoppage problem. But, it could also be caused by air being forced out of the tub drain because of a stoppage. Your problem could be caused by NO vent, an IMPROPER vent, or a PLUGGED vent. The vent COULD be plugged with frost and you would not be able to see it without looking INTO it.

Thanks guys. I would tend to think that it is flushing "good". Nothing has changed in years so its not "no"vent or "improper" vent. Perhaps frost plug. I guess as long as it flushes well all is good.
Thanks for your time both of you. I do appreciate it!

FWIW, almost EVERYTHING will drain better without a vent. The vent is to keep it from draining so good that it siphons the water out of the trap. I once had a customer who wanted me to connect his tub drain to the stack. He had a 10' pipe dropping out of the tub all the way to the basement where he wanted me to make the connection. I told him that if I made that connection, the tub would drain so "good" that it would suck everything in the room, including the baby, down the drain when he pulled the plug. A bit of hyperbole, but he got the message.